Differentially expressed circular RNAs and their functional enrichment analysis
A large body of knowledge has emphasized the significant role of noncoding RNAs in sheep fertility [
32,
62‐
64]. The study of ceRNA consider landmark in understanding the mutual regulatory relationship and interactions of RNA-RNA [
64,
65]. In this study, high-throughput sequencing was used to identify and characterize ovarian tissue circRNAs from Small Tail Han Sheep (X_LC) and Dolang sheep (D_LC) and determined ceRNA regulatory network. In this study, majority of circular RNAs were predicted with exon 1–5 which are consistent with the sheep uterus circRNAs [
27] but inconsistent with sheep pituitary gland circRNAs [
26] thereby exhibit complexity and functional diversity. Besides, studies have been reported about the expression and potential biological functions of circRNAs in reproductive organs in goats [
66], mice [
67] and humans [
68]. Several findings have been reported about the circRNAs expression and their biological functions as miRNA sponge [
25,
27,
28]. In sheep uterus study, 147 and 364 circRNAs out of 32,687 were differentially expressed in polytocous and monotocous groups in the follicular phase and luteal phases and DECs host genes significantly enriched with estrogen signaling pathway, oxytocin signaling pathway [
27]. While, in the comparative study between the follicular phase/luteal phase of sheep, 15 DECs out of 3223 were predicted and DECs host genes were enriched in the Rap1 signaling pathway, PI3K–Akt signaling pathway and neuroactive ligand–receptor interactions [
28]. In this study, we found 9,878 new circRNAs with a total length of 23522667nt and an average length of 2381.32nt respectively. 44 circRNAs were differentially expressed and majority of enriched GO terms of DECs host genes were related to cell proliferation and reproductive process and KEGG enrichment analysis enriched in cell adhesion molecules, phagosome, PI3K-Akt signaling pathway, neuroactive ligand-receptor interaction, glutathione metabolism, metabolism of xenobiotics by cytochrome P450, MAPK, axon guidance, valine, leucine and isoleucine degradation, and endocytosis pathways. A similar phenomenon was observed in other studies [
27,
28,
32]. Metabolic changes reported during transition period from ovulation to the estrous and biomolecules including vitamins, amino acids, lipid, benzoic acid, carbohydrates and other intermediate and secondary metabolites are at their highest levels at the time of ovulation [
27,
69,
70]. Prior stud have been reported that MAPK pathway have a great effect on granulosa and cumulus cells which plays essential role in oocyte maturation [
71]. Therefore, based on above evidences we suggested that DECs might contribute in sheep prolificacy.
CeRNA analysis and networking/ functional enrichment analysis of ceRNA regulatory genes
Circular RNAs (circRNAs) as ceRNA contribute to a various signaling pathways that are crucial for the development process [
72]. The fact that mRNA expression negatively regulated by miRNA activity, whereas, circRNAs inhibit or relieve repression of miRNA for translation [
73]. Study determined the role of circRNA as ceRNA in follicular development in Hanper sheep [
32]. Here, circRNA and mRNA target binding sites were predicted for miRNA (Fig.
3A-B and Table
2) which benefited to determined regulatory relationship among differentially expressed circRNAs, miRNA, mRNA (ceRNETs) (Fig.
5A). In this study, numerous circRNAs holding binding site for common miRNA such as; circRNA_9564, circRNA_8312 shared novel339_mature miRNA, circRNA_9564, circRNA_8401, circRNA_3257 shared novel579_mature miRNA respectively. Whereas, circRNA_8396, circRNA_9690, circRNA_4140 have potential binding sites for only single miRNA including novel339_mature, novel62_star, and novel34_mature > novel661_mature respectively. novel579_mature dominantly target to the number of mRNAs including; CHMP4C, EPHA3, SCML4, MSH4, LOC106991756, LOC101120875, HOXA11, RGS5, SLC45A1, C6H4orf32, LOC101104325, and PDK4 which significantly involved in pathways including endocytosis and axon guidance pathway. Prior study determined the crucial role of endocytic pathways in the development of the reproductive organs [
74]. circRNA_8396 have potential binding sites for novel339_mature which target to the following mRNAs; CENPF, CLEC19A, AURKB, and LOC101102473.circRNAs_4140 have potential binding sites for novel34_mature > novel661_mature which target to KCNK9. To further investigate potential circRNAs mechanisms of action in ceRNA regulatory network, we applied KEGG analyses. It was reported that foxO signaling pathway, cell cycle, p53 signaling pathway, endocytosis, progesterone-mediated oocyte maturation participate in reproduction [
75]. The results we obtained suggest that multiple signaling pathways form a complex regulatory network involved in prolificacy.
Coherence of ceRNA regulatory network with differential expression datasets to regulate reproduction
CircRNAs biological function as a microRNA (miRNA) sponge and regulating the target mRNA expression by forming the circRNA-miRNA-mRNA regulatory axis [
60]. Pearson correlation based networking and expression profiling provided ceRNA regulatory pairs as shown in figure (Fig.
5A). KEGG enrichment analysis of mRNA involved in ceRNETs revealed significant pathways such as; Galactose metabolism, Glutathione metabolism, Metabolism of xenobiotics by cytochrome P450, Drug metabolism—cytochrome P450, Cell adhesion molecules (CAMs), Antigen processing and presentation, p53 signaling pathway, and Progesterone-mediated oocyte maturation pathways respectively. Of note, studies have demonstrated the involvement of above-mentioned pathways in ovarian physiology and play essential role in follicular development, oocyte maturation, development of reproductive organ, proliferation, immunity, antioxidant, and metabolic process [
74‐
77]. Previous studies demonstrated that the exposure of xenobiotics may destroy the primordial follicles which is responsible for premature ovarian failure and reduce fertility [
78,
79]. Glutathione metabolism, well known for their spectacular role as a free-radical scavenger, intervenient in xenobiotics metabolism, cell-cycle regulation, and a reservoir of cysteine [
80], as well as play key roles in cellular redox homeostasis [
81]. Study reported the involvement of Progesterone-mediated oocyte maturation pathways in oocyte development in sheep breeds [
82],and regulating the uterine receptivity and maintenance of pregnancy [
83].
Researchers established the role of galactose metabolism in energy delivery, and galactosylation of complex molecules [
84]. The metabolism of galactose to UDP-glucose involves three major enzymes, galactokinase, galactose-1-phosphate uridyltransferase (GALT), and UDP-galactose-4-epimerase. In case of any disturbance or deficiency of UDP-galactose are considered to be important in the pathogenesis of ovarian dysfunction and in GALT deficiency [
85] due to interference with ovarian apoptosis and gonadotrophin signaling, thus effecting fertility. Wu et al. reported that deregulation of glucose metabolism in diabetic individuals [
86], which might induce ovarian anomalies [
87]. It is believed that these pathways may play a critical role in regulating ovarian physiology. Previously, study have also examined miRNAs as important components of the p53 transcriptional network [
88]. MiR-25 and miR-30d, have been shown to negatively regulate the transcription of P53 gene. Several other miRNAs, including miR-16–1, miR-143, miR-145, miR-34, miR-194, miR-192, miR-215, and miR-29, have been identified as miRNAs that are involved in the transcription P53 network, either by being directly altered by p53 or through their associations with downstream genes targeted by p53[
89].
Significance of ceRNA target gene in regulating reproduction and associated metabolic syndrome
Through ceRNA circRNA-miRNA-mRNA interaction analysis and based on functional enrichment analysis established the following key ceRNA regulatory axis; circRNA_3257-novel579_mature-EPHA3, circRNA_8396-novel130_mature-LOC101102473, and circRNA_4140- novel34_mature > novel661_mature-KCNK9. These shortlisted circRNAs were the result of sense-overlapping and the length of circRNA_3257, circRNA_4140, circRNA_8312, circRNA_8396, circRNA_9690, circRNA_9564 were 758, 3145, 33,662, 44,924, 1793, and 76,127 nt respectively. We found that downregulated circRNA_3257 had a potential binding site for upregulated novel579_mature miRNA and influences EPHA3 gene expression, which is regulated by the protein kinase A (PKA) pathway [
90]. The ephrin-Eph gene family has a known physiological role in regulating mammalian reproductive function, such as in granulosa cells of bovine ovarian follicles [
91], mouse [
90], and human luteinizing granulosa cells [
92]. Prior studies have suggested that EPHA3 plays a role in treating ovarian endometriosis, potentially promoting apoptosis and autophagy of macrophages via the inhibition of the mTOR signaling pathway and reducing oxidative stress [
93,
94]. In this study, we found that downregulated EPHA3, involved in the axon guidance pathway, functions through cell-to-cell contact [
95], and regulates the expression of guidance proteins Ras and Rho GTPases during embryonic development [
96]. Furthermore, EPHA3 has been shown to interact with presenilin genes sel-12 (PS1), regulating axon guidance and kinesin-mediated axonal transport of motor neurons. Loss of function mutations in the presenilin genes sel-12 results in abnormal axonal projections, an effect attributed to altered Notch signaling pathway [
97], which is integral to maintaining fertility in the ovaries through developmental regulation and granulosa cell function [
98]. Based on the ceRNA hypothesis, we suggest that circRNA_3257 may act as a sponge for novel579_mature miRNA, thus favoring the expression of repressed EPHA3, and potentially play a role in the fecundity of Small Tail Han Sheep.
LOC101102473 is a target gene of novel130_mature that is downregulated in ovary tissue of Small Tail Han Sheep [
32], and is involved in pathways related to reproduction, such as FoxO signaling pathway, Cell cycle, p53 signaling pathway, Progesterone-mediated oocyte maturation, consistent with the previous research [
77]. FOXO signaling is the central pathway controlling growth and metabolism in all cells [
99], and closely related with ovarian function [
100]. Low levels of p53 expression maintain cell cycle homeostasis and cell death [
101]. Dysregulation of the cell cycle, particularly the G1-S-phase transition, is implicated in epithelial ovarian cancer [
102]. If DNA damage occurs, p53 accumulates in the cells and induces p21-mediated inhibition of cyclinD/CDK. The transition to S phase is triggered by the activation of the cyclinD/CDK complex, which phosphorylates the retinoblastoma protein pRb, a known cell proliferation regulator [
102]. The transcription factor p53 functions as a suppressor of cell growth, and alterations in p53 lead to loss of this negative growth regulation and more rapid cell proliferation. Previous studies have demonstrated that dysfunction of the p53 signaling pathway contributes to the development of ovarian cancers [
103], hence affecting fertility. These outcomes suggest that circRNA_8396 might affect sheep fecundity by regulating the expression of LOC101102473 genes linked with the above signaling pathways via sponging novel130_mature miRNA.
The gene KCNK9, which is linked to potassium channels in X_LC-vs-D_LC and has been previously identified in the membrane of cow oocytes [
104], is upregulated as a target gene of novel34_mature > novel661_mature. While previous studies have shown its potential role as a therapeutic target for adenomyosis which induce infertility [
105]. KCNK9 also helps to control progesterone production in the ovary, and intracellular potassium and calcium concentrations have a significant impact on fertility [
106]. Our functional enrichment analysis has revealed that KCNK9 is involved in the aldosterone synthesis and secretion pathway, which is critical for maintaining blood pressure, circulating blood volume, and uteroplacental perfusion during pregnancy [
107]. Several other studies have shown that aldosterone synthesis and secretion levels are high throughout pregnancy, indicating potential involvement in the regulation of placental and fetal development [
108,
109]. However, studies have also reported that aldosterone is involved in gynecological diseases due to metabolic alterations induced by the usage of hormonal contraceptives, PCOS, uterine fibroids and endometriosis, inflammation, and hypertension, which can result in an increase in the synthesis of angiotensinogen, activating all the RAAS and inducing the onset of sodium and water retention [
110]. Therefore, we hypothesize that circRNA_4140 may act as a sponge for novel34_mature > novel661_mature and regulate the expression of KCNK9, which plays a crucial role in fertility.
In addition to the upregulated novel339_mature, the downregulated LOC101110545 (HLA class II histocompatibility antigen, DO alpha chain) gene is also involved in reproduction, as it is a target gene involved in the Cell adhesion molecules (CAMs) [
111] and Phagosome [
112,
113] pathways crucial for reproduction. CAMs are a family of glycoproteins that play a crucial role in various physiological processes, including cell migration, tissue development, and immune response. They also play key roles in inducing leukocyte infiltration in an inflammatory site during ovulation [
114]. Studies have shown that CAMs are implicated in the regulation of ovarian follicular development, oocyte maturation, and fertilization in sheep [
115,
116], all of which are critical for successful reproduction and fecundity. One of the key CAMs involved in sheep reproduction is integrin, which facilitates cell–cell and cell–matrix interactions, and is regulated during folliculogenesis and ovulation. Integrin-mediated signaling is essential for the survival and growth of ovarian follicles, as well as for the adhesion and migration of oocytes and granulosa cells during follicle development [
115]. Another important CAM in sheep fecundity is selectin. Selectin is another important CAM in sheep fecundity. It is a family of carbohydrate-binding proteins that mediate leukocyte-endothelial cell adhesion during inflammation and immune responses. Studies have reported that selectins play a role in follicular development and ovulation by facilitating the adhesion and migration of leukocytes to the ovary and promoting the release of inflammatory cytokines, chemokines and growth factors [
117,
118]. Changes in these molecules are associated with the cyclic changes in the estrous cycle, compounding their role in the ovulatory process [
119]. Further research is needed to elucidate the precise mechanisms underlying CAM-mediated signaling in sheep reproduction and to develop novel interventions to improve sheep breeding and production.
In livestock animals, including sheep, phagosomes play a crucial role in regulating reproductive processes such as ovulation, fertilization, and embryo development [
120]. As ovarian follicles develop, some follicles undergo atresia, a process by which they degenerate and are eliminated from the ovary [
121,
122]. This process involves the death of granulosa cells, which are engulfed and cleared by phagocytes via phagocytosis. Moreover, in mammals, the engulfment of apoptotic cells by macrophages induces the production of anti-inflammatory cytokines to suppress inflammatory responses [
123]. Failure to properly clear apoptotic granulosa cells can lead to inflammation and oxidative stress, which can negatively impact follicular development and ovulation [
122,
124]. Based on the evidence presented, we can infer that circRNA_8312 may influence LOC101110545 gene expression by suppressing novel339_mature activity. Although the role of the LOC101110545 gene in reproduction has yet to be investigated, our data suggest its possible involvement in reproductive performance through ceRNA networks that regulate ovarian physiology. Comparison between our results and those of previous studies suggests that these selected circRNAs may act as ceRNAs and contribute to signaling pathways regulating reproductive traits. However, further research is required to identify the exact mechanisms involved. This study provides an important source of information that various circular RNAs can act as competitive endogenous RNA, containing common miRNA binding sites, and may act as miRNA sponges to regulate the expression of valuable prolificacy genes, thereby improving sheep fecundity.